108 



THE ECOLOGICAL RELATIONS OF ROOTS. 



from their point of origin and are reenforced by a great network of branches 

 arising from the older roots in the deeper soil at 6 to 10 inches. The deeper 

 roots and their network of fine branches spread widely, often running for 

 several inches horizontally through the moist gravel soil. The whole root 

 system is surprisingly well developed, considering the habitat conditions 

 under which the plant grows. 



FOREST ROOT SYSTEMS AND THE FOREST ENVIRONMENT. 



A survey of the preceding data reveals the fact that the herbs and 

 shrubs of the forest floor are relatively shallow-rooted. Almost with- 

 out exception the bulk of the absorbing system lies within the surface 

 18 inches of soil. Arctostaphylos and Rosa have roots which penetrate 

 deeper. The former, however, shows its xerophytic tendencies by 

 being the typical half-shrub of the open pine forest and often forming 

 large mats in the half-gravel-slide. In the Douglas fir forest it occurs 

 only in the most open places and disappears entirely in the spruce. 

 Rosa is a relict from an earlier stage of succession. It makes a much 

 better growth in full light, even in dry situations, where it forms dense 

 thickets. Even the roots of many of the Douglas fir and spruce trees, 

 including seedlings, saplings, and mature plants which were inciden- 

 tally encountered in the course of the work, were found to possess 

 many shallow roots (plate C ) . The root relations of forest dominants 

 offer an exceedingly profitable field for investigation. 



The water-content of the soil offers the logical explanation for the 

 shallow root development of these forest plants. The forest soil is 

 usually deeper and always much richer in organic matter than that 

 of the half-gravel-slide. Usually there is a layer of 1 or 2 inches of 

 duff, beneath which the soil is very rich in decomposed humus to a 

 variable depth of 8 to 18 inches or more. Such a substratum furnishes 



Table 20. — Average daily evaporating power of the air in the forest communities. 



Date. 



June 10-17 



June 17-24 



June 24 to July 1 



July 1-8 



July 8-15 



July 15-22 



Douglas 

 fir. 



c.c. 

 28.8 

 11.1 

 20.4 

 15.2 

 5.7 

 6.1 



Engelmann 

 spruce. 



c.c. 

 21.6 



7.1 

 15.0 

 11.1 



4.4 



5.2 



Date. 



July 22-29 



July 29 to Aug. 5 



Aug. 5-12 



Aug. 12-17 



Aug. 17-23 



Douglas 

 fir. 



c.c. 

 11.3 

 16.0 

 8.6 

 8.5 

 11.6 



Engelmann 

 spruce 



c.c. 

 8.5 



11.4 

 6.2 

 6.3 

 8.8 



an excellent medium with a high water-holding capacity to catch the 

 precipitation of winter as well as that of the frequent summer showers. 

 The shade of the trees and various forest layers reduces the evaporat- 

 ing power of the air (table 20), while the water-loss from the soil is 

 further retarded by the layer of duff. 



